Blender addons are called also 3d party plugins?
A third-party software component is a reusable software component developed to be either freely distributed or sold by an entity other than the original vendor of the development platform. Blender is an example of this.

So it has 3rd party plugins because it's a 3rd party software. Another way of saying these are Blender add-ons because it's more particular. Having two different names for the same thing is normal.